Photolithographic tools are common place in the microelectronics industry. In a photolithographic tool a mask is placed at an object plane of the lens. The mask is potentially non-transmitting to the frequencies of electromagnetic radiation used in the tool and has a predetermined pattern which transmits these frequencies. An image of the mask is produced by the photolithography lens onto a substrate which is generally a resist covered electronics chip or chip packaging substrate. Those regions of the resist which are exposed to the image forming radiation have a latent image of the mask pattern formed therein. Either the exposed or non-exposed regions of the resist are removed by development process to form a resist mask through which the substrate is etched to form a pattern in the substrate. Also, the photolithographic system can be used to directly ablate the image of the mask onto an ablatable surface. The ability to directly ablate a pattern onto a surface reduces the cost of fabrication since there can be fewer process steps, for example, the steps in depositing and removing a resist mask are eliminated. The optical system according to the present invention permits the direct ablation of a mask pattern onto an ablatable surface, in particular, a polyimide surface.
Commonly used photolithographic tools generally use a system of lenses which rotates the image 180.degree. relative to the object. In some photolithographic tools, the object field contains the total object which is to be imaged onto a substrate. This image of the entire object can then be stepped across the substrate to form repeated pattern of the object. If the object is be imaged is larger than the image field of the lens, to completely print the object requires that the object and the image field be moved in the opposite direction since the image plane is 180.degree. rotated with respect to the object plane. This is a relatively complex procedure since the image and the object cannot be held fixed in position relative to each other. One way to avoid this is to add to the photolithographic system another complete set of optics which will rotate the image another 180.degree., the final image being rotated 360.degree. with respect to the object. With such a double sized optical system, where the object to be imaged is greater than the image field of the system, the object plane and the image plane can be fixed relative to each other and scanned relative to the lithographic system to form a complete image of the object. However, adding a duplicate set of lenses to rotate the image another 180.degree. substantially increases the cost complexity and size of the photolithographic system.
Applicants have discovered a new projection lens which also permits scanning the image and object fields together without requiring a substantial increase in the complexity of the system.
One use of applicant's lenses is for a system to ablate the image of the object mask onto an ablatable surface. Laser tools for milling a work piece are known in the prior art.
It is an object of the present invention to provide an image at the image plane having the same orientation as the object and to provide an apparatus which can scan the object and image together.
U.S. Pat. No. 4,749,840 to Piwczyk describes an optical system using lenses and a flat mirror for ablative photodecomposition of a polymer surface using a beam of ultraviolet light from an excimer laser. The beam is aligned to the location to be ablated by irradiation with visible light which is optically observed through an eye piece. Pickwick does not use curved reflecting surface having substantially coincident axes as in the present invention. Moreover, the alignment of the workpiece is by visual observation. In contradistinction, the apparatus of the present invention uses an alignment beam which does not have to be visually observable.
U.S. Pat. No. 4,584,455 to Tomizawa describes a laser beam irradiating apparatus which includes a working laser beam for machining a workpiece coupled to a visible laser beam which is made coincident therewith. The visible laser beam is detected by a camera for positioning the workpiece. The apparatus contains planar reflecting surfaces but not curved reflecting surfaces. The visible laser beam is monitored by a camera to determine the location of the machining beam on the workpiece.
U.S. Pat. No. 3,689,159 to Taniguchi et al. describes a laser shaping apparatus for automatically shaping a workpiece with a laser and for providing automatic controls so that the focal point of the laser light which is focused by a lens always corresponds with point on the workpiece which is being shaped.
U.S. Pat. No. 3,951,546 to Markle describes a IX projection lens which permits scanning the image and object fields together. Markle describes a three-fold mirror array for scanning projection system having an image orientation for a scanning and yielding an image symmetry identical to that obtained in contact printing. The three fold mirror array is provided with a pair of mutually perpendicular reflecting plane faces which constitute a roof. The line of intersection of the roof surfaces is perpendicular to the third planar reflecting surface of the three fold mirror combination. The system contains two concentric curved mirrors, one of which is concave, the other of which is convex. An object, e.g. a mask, is illuminated. The light goes through the mask onto the third planar surface from which it reflects to the concave mirror, from which it reflects into the convex mirror, from which it reflects back to the concave mirror, from which it appears to straddle the dihedral angle of the roof mirror, from which it reflects to the image plane, e.g. a semiconductor wafer. The Markle system has an even number of reflections, and the planar mirror surfaces are mutually orthogonal so that the object and image planes are parallel, therefore, the mask image at the image plane has the same orientation as the mask at the object plane. The mask and wafer can be scanned together. The combination of the three-fold mirror, concave mirror and convex mirror form a IX projection lens for the Markle scanning projection system. The Markle lens uses the Offner lens which is the concave/convex mirror combination of U.S. Pat. No. 4,241,390 to Markle and Offner. The Offner lens is not corrected for an axis aberration, but is corrected on a specific annular field. Good imaging at the image plane occurs only in a thin anular region In contradistinction, the lens of the present invention is corrected for aberrations, in a region including and around the axis. The region of good imaging according to the present invention is not limited to an annular field. To achieve the same useful region of good imaging using the Offner system requires that the Markle lens be substantially increase in size so that the useful area of the present invention fits into the annular ring of the Markle lens. This situation is schematically shown in FIG. 1. Region 1, shown as a circle, represents the field of good imaging of the lens system of the present invention. To achieve this same region the Markle lens system will have to have annular region 3 as the region of good imaging. This requires substantially large optical elements. Region 1 is not limited to a circular region, but can be square, rectangular, triangular, or any other geometrical figure that fits in the region of aberration correction.
It is another object of the present invention to provide a projection lens for which the region of good imaging is not limited to an annular region.
It is another object of the present invention to provide a projection lens having a large area of good imaging.
It is another object of the present invention to provide an apparatus for ablating in a substrate surface an image having the same orientation as the object projected by the apparatus.
It is another object to provide an ablation aperture which aligns mask to substrate with one wavelength and ablates with another wavelength of radiation.
These and other objects and features of this invention will become apparent from the following more detailed description of the preferred embodiments and the figures appended thereto.